A joining unit of this type is known from DE 203 05 789 U1.
The joining unit described therein contains a spindle drive for the generation of a linear movement of a joining tool. The spindle drive is formed by a hollow shaft motor that encloses a spindle.
The hollow shaft motor is embodied as an electric drive and encompasses a stator as well as a rotor which is firmly attached to the spindle. The rotor causes the spindle to execute a rotational movement.
For the generation of the linear movement of the joining tool a plunger is provided that forms the lower surface of a tubular housing insert having an interior space through which a portion of the spindle extends. A nut is provided at the upper side of the housing that engages a thread at the lateral surface area of the spindle. Due to this coupling, the rotational movement of the spindle is directly converted into a longitudinal movement of the plunger and, therefore, of the joining tool.
It is an advantage of this joining unit that the conversion of the rotational movement of the spindle drive into a linear movement does not require any gearing.
Therefore, the joining unit shows a simple and cost-effective construction.
It is an essential disadvantage of such joining units that the functional control thereof is primarily performed via external units. Although in the joining unit itself are provided position sets which control the time course of the velocity of the joining tool in accordance with target positions relative to the workpiece to be processed, it is necessary to perform the activation of the respective position set via an external unit such as for example an SPS control.
Furthermore, using such external units it is known to perform a power cut-off as a protection against damages of the joining tool or of the workpiece. In a power cut-off of this type an emergency shut-off of the joining unit is actuated via the external unit if a maximum allowable force is exceeded that acts on the spindle. Due to the time required for the processing of the signal in the external unit as well as the time for signal transmission between the joining unit and the external unit, it may be possible that the emergency shut-off is not actuated in time thus resulting in damage to components of the joining unit due to excessive straining by force.
Moreover, a system in which essential control functions are performed by external units is relatively complex and, therefore, difficult to handle by the user.